This invention relates to use of a particular catalyst composition comprising a thermally stable layered metal chalcogenide, such as a titanium oxide, having adjacent layers separated by metal chalcogenide pillars, such as a silicon oxide, and an alkali metal, such as lithium or rubidium, for selective conversion of feedstock comprising C.sub.n paraffins, such as methane, to product comprising C.sub.n+1.sup.+ hydrocarbons plus C.sub.2n dimers. For purposes of describing the present invention herein, the term "metal" is considered to include the elements boron, silicon, phosphorus and arsenic.
The conversion of methane into more valuable chemicals has long been a challenge to chemists and engineers alike. Methane is, in general, much more stable than its derivatives. It is especially difficult to selectively oxidize methane without conversion to carbon oxides and water. G. E. Keller et al, J. of Catal., 73, 9-19 (1982) reported converting CH.sub.4 to C.sub.2 H.sub.4 by sequential reaction with pulses of N.sub.2, air, N.sub.2, etc., to avoid deep oxidation. With Sb, Sn, Mn oxides as catalysts, conversion of less than 10% CH.sub.4 has been obtained. T. Ito et al, J.A.C.S. 1985, 107, 5062, reported less than 10% conversion of CH.sub.4 to C.sub.2 H.sub.4 using a Li on MgO catalyst. U.S. Pat. No. 4,443,649 teaches that substantial amounts of C.sub.2 H.sub.4 can be obtained in addition to carbon oxides by contacting methane with Mn oxide on SiO.sub.2 at about 800.degree. C. The catalyst aged rapidly (within minutes) due to depletion of oxygen. K. Otsuka et al, Chem. Lett. 467 (1986) reported methane oxidation to ethylene and other products over lithium impregnated metal oxides.
Oxidative coupling of methane over LaAlO.sub.3 catalyst was reported by H. Imai and T. Tagawa in J. Chem. Soc., Chem. Commun., 52-53 (1986). K. Otsuka, Q. Liu and A. Morikawa reported synthesis of ethylene by partial oxidation of methane over LiCl-Sm.sub.2 O.sub.3 catalyst to produce C.sub.2 compounds, i.e. ethane and ethylene in J. Chem. Soc., Chem. Commun., 586-587 (1986). Oxidative dimerization of methane over BaCO.sub.3, SrCO.sub.3 and same promoted with alkali was reported by K. Aika et al in J. Chem. Soc., Chem. Commun., 1210-1211 (1986). U.S. Pat. No. 4,574,038 shows a process for converting methane to ethylene and hydrogen over a metal powder catalyst, the methane and catalyst subjected to microwave radiation.
It has now been found that thermally stable layered metal chalcogenides containing metal chalcogenide pillars separating the layers which have been composited with an alkali metal by way of, for example, impregnation with an alkali metal halide, may be employed to selectively convert C.sub.n paraffins to C.sub.n+1.sup.+ hydrocarbon product comprising dimers of the C.sub.n paraffin by the mechanism of oxidative coupling.
Many layered materials are known which have three-dimensional structures which exhibit their strongest chemical bonding in only two dimensions. In such materials, the stronger chemical bonds are formed in two-dimensional planes and a three-dimensional solid is formed by stacking such planes on top of each other, the interactions between the planes being weaker than the chemical bonds holding an individual plane together. The weaker bonds generally arise from interlayer attractions such as Van der Waals forces, electrostatic interactions, and hydrogen bonding. In those situations where the layered structure has electronically neutral sheets interacting with each other solely through Van der Waals forces, a high degree of lubricity is manifested as the planes slide across each other without encountering the energy barriers that arise with strong interlayer bonding. Graphite is an example of such a material. The silicate layers of a number of clay materials are held together by electrostatic attraction provided by ions located between the layers. In addition, hydrogen bonding interactions can occur directly between complementary sites on adjacent layers, or can be provided by interlamellar bridging molecules.
Laminated materials such as clays may be modified to increase their surface area. In particular, the distance between the layers can be increased substantially by absorption of various swelling agents such as water, ethylene glycol, amines and, ketones, which enter the interlamellar space and push the layers apart. However, the interlamellar spaces of such layered materials tend to collapse when the molecules occupying the space are removed by, for example, exposing the clays to high temperatures. Accordingly, such layered materials having enhanced surface area are not suited for use in chemical processes involving even moderately severe conditions.
The extent of interlayer separation can be estimated by using standard techniques such as X-ray diffraction to determine the basal spacing, also known as "repeat distance" or "d-spacing". These values indicate the distance between, for example, the uppermost margin of one layer with the uppermost margin of its adjoining layer. If the layer thickness is known, the interlayer spacing can be determined by subtracting the layer thickness from the basal spacing.